A press cushion of this type is known from EP 0 235 582 A2. The known press cushion shall be used in particular for producing multi-layer and flexible circuit boards for printed electronic circuits in so-called high-pressure presses. Press cushions for producing materials for electronic circuit boards are produced using high pressures (up to approximately 1000 N/cm2 or 100 bar) and also high temperatures (up to approximately 220° C.) through a lamination process. The sandwich shaped press cushions include a reinforcement layer made from fibers, for example in the form of a central glass fiber woven material in order to reduce the deformation during press operations and also when removing the finished pressed product and when inserting a new press blank to increase the service life of the press cushion. Furthermore press cushions for printed circuit board production are typically configured with anti-adhesion layers on their surfaces, for example in the form of a foil made from polytetrafluorethylene (PTFE). Furthermore it is important to prevent an abrasion of fibers or particles from the press cushion during operations of high pressure press cushions of this type since abrasion of this type would contaminate the circuit board material and would thus lead to a degraded quality or even technical non-usability of the circuit board material.
Press cushions are furthermore often used for producing wood materials with coated surfaces. Laminates of this type are typically used for furniture production and as flooring materials. Thus decorative layers in the form of abrasion resistant plastic foils with various decors (imitation wood, imitation tile, imitation stone) are applied, for example, to wood fiber boards (“particle boards” or multidirectional fiber (MDF) boards). The connection between the decorative layer and the carrier material of the fiber board is typically provided using melamine resin which is cured under pressure and temperature (up to 50 bar or 500 N/cm2 pressing pressure and up to 220° C. press temperature) in a laminating press.
When using press cushions for circuit board production and also when applying a decorative layer to fiber materials, the purpose of the press cushions is to compensate thickness or height tolerances in the hydraulic press. Thickness or height tolerances of this type result from dimensional tolerances in the press blank (fiber board or decorative foil or conductor base material), however, in particular in the press itself. Thus, press units typically include heatable press plates with dimensions of several meters so that besides production tolerances also problems occur through plate warping through different temperature distributions during press operations. Also for so-called press plates which come in contact with the press material on both sides dimensional tolerances can occur. Thus, an important feature of press cushions is compensating such dimensional tolerances in the system “heat plate—press plate—press blank—press plate—heat plate.” In order to permanently provide a thickness compensation of this type without damaging the press cushion the press cushion has to have sufficient elastic properties, this means spring elastic properties which bring it back again into its original shape independently after unloading the pressure. Typically elastomeric materials are used for the known press cushions in order to obtain the desirable spring properties. In particular silicone elastomeric materials, fluoro-elastomeric materials or fluoro-silicone elastomeric materials are being used for press cushions due to their favorable temperature stability and their resistance against various chemical substances.
Due to the good heat conductivity required for press cushions which facilitates the necessary fast heat transfer from the heat plate through the press cushion and the press plate into the press blank, the press cushions typically include components made from materials with good heat conductivity, in particular metals. Typically copper and/or brass and/or stainless steel and/or other metal alloys are used for this purpose.
The known press cushions can be divided in principle into press cushions including textile flat materials and press cushions which include a closed material layer. A press cushion with a closed material layer is known for example from DE 23 19 593 A or EP 1 300 235 A. In order to provide reliable integrity of the press cushions also under high pressing pressures and during handling a woven support material for example made from bronze fibers is embedded in the silicon elastomeric material that is being used. In order to improve heat conductivity the press cushion according to EP 1 300 235 A includes a surface overhang of the metallic support woven material beyond the elastomeric material. In order to further increase heat conductivity particles with high heat conductivity (for example, metal powder or silica powder) can be introduced into the elastomeric material.
As recited supra, many press cushions are flat textile materials which can be configured in particular configured as woven materials, knitted materials or fleece materials. This way, based alone on the geometry of the press cushion or of the threads processed therein a good shape change capability and therefore a good thickness compensation is assured.
This applies in particular when the threads have high indigenous elasticity in a direction transversal to their longitudinal axes, in particular when they are made from elastomeric material or have a tension resistant core (for example, made from metal or a synthetic fiber) which is encased in elastomeric material. Press cushions of this type are known for example from EP 0 735 949 A, EP 1 136 248 A and EP 1 779 999 A in the form of woven press cushions. Press cushions based on knitted materials are disclosed in particular in EP 1 033 237 A or EP 1 040 910 A and EP 1 040 909 A. In order to provide particularly good point elasticity to the press cushion, it is known from EP 1 386 723 A to use jacketed fibers alternating with the elastomeric material, wherein the elasticity of the fibers transversal to the longitudinal axis of the thread alternates between being higher or lower. Furthermore press cushions or press cushion—press plate combinations are know in which a coherent cavity of the press cushion or of the combination is filled with a material that is flow-capable at the operating temperature of the press (EP 2 189 276).
A disadvantage of the mat-shaped press cushions according to DE 23 19 593 A is their comparatively high stiffness and lack of point elasticity. On the other hand side the press cushions which are configured as flat textile materials are comparatively complex to produce. This applies in particular when jacketed threads are being used, possibly with different types, wherein the material of the core has to be provided with the elastomeric jacket material during an extrusion process in a first step. The textile processing that is performed in a second step (weaving or knitting) is very complex again, since the processed elastomeric threads or metal threads contrary to classic textiles cause a substantially more difficult and slower manufacture.